CSIRO used LCA methodology to determine cradle-to-farmgate GHG emissions for rainfed wheat grown in monoculture or in sequence with the break crops canola and field peas and for the break crops, in the south-eastern grains region of Australia.
Total GHG emissions were 225 kg carbon dioxide equivalents (CO2-e)/t grain for a 3 t/ha wheat crop following wheat, compared with 199 and 172 kg CO2-e/t for wheat following canola and field peas, respectively. On an area basis, calculated emissions were 676, 677 and 586 kg CO2-e/ha for wheat following wheat, canola and field peas, respectively. Highest emissions were associated with the production and transport of fertilisers (23–28% of total GHG emissions) and their use in the field (16–23% of total GHG emissions). Production, transport and use of lime accounted for an additional 19–21% of total GHG emissions.
The lower emissions for wheat after break crops were associated with higher yields, improved use of fertiliser nitrogen (N) and reduced fertiliser N inputs in the case of wheat after field peas. Emissions of GHG for the production and harvesting of canola were calculated at 841 kg CO2-e/ha, equivalent to 420 kg CO2-e/t grain. Those of field peas were 530 kg CO2-e/ha, equivalent to 294 kg CO2-e/t grain. When the gross margin returns for the crops were considered together with their GHG emissions, the field pea–wheat sequence had the highest value per unit emissions, at AU$787/t CO2-e, followed by wheat–wheat ($703/t CO2-e) and canola–wheat ($696/t CO2-e).
Uncertainties associated with emissions factor values for fertiliser N, legume-fixed N and mineralised soil organic matter N are discussed, together with the potentially high C cost of legume N2 fixation and the impact of relatively small changes in soil C during grain cropping either to offset all or most pre- and on-farm GHG emissions or to add to them.
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